92 CONTEMPORARY SCIENCE 



cultures of these cells living on the extracts from chick 

 embryos have been kept alive now for seven years. 



All this points to the idea that death is not inherent in 

 the individual cell, but is only the fate of more compli- 

 cated organisms in which different types of cells or tissues 

 are dependent upon each other. In this case it seems to 

 happen that one or certain types of cells produce a sub- 

 stance or substances which gradually become harmful 

 to a vital organ like the respiratory center of the medulla, 

 j or that certain tissues consume or destroy substances 

 which are needed for the life of some vital organ. The 

 mischief of death of complex organisms may then be 

 traced to the activity of a black sheep in the society of 

 tissues and organs which constitute a complicated multi- 

 cellular organism. 



IV 



While in human beings there is no sharp limit between 

 youth and maturity, in many insects and amphibians this 

 limit is marked by a sudden metamorphosis in the shape 

 of their body. The frog hatches from the egg as a tad- 

 pole without legs and with a long tail. After a certain 

 length of time legs begin to grow, the tail disappears, the 

 form of the head and mouth change, the skin looks dif- 

 ferent, and the tadpole is transformed into a frog. It 

 is possible that some of the changes underlying metamor- 

 phosis are due to changes in the circulation of the blood. 

 Gudernatsch made the remarkable discovery that this 

 metamorphosis, which in our climate usually occurs dur- 

 ing the third or fourth month of the life of the tadpole, 

 can be brought about at will even in the youngest tad- 

 poles, by feeding them with thyroid gland, no matter from 

 which animal. By feeding very young tadpoles with this 

 substance, frogs not larger than a fly could be produced. 

 Allen added the observation that if a young tadpole is 

 deprived of its thyroid gland, it is unable ever to become 

 a frog; and that it remains a tadpole which can reach, 



